Reference is made to three copending applications (U.S. patent application Ser. No. 830,166, "Scanning Receiver Allocation Method and Apparatus for Cellular Radiotelephone Systems", by Menich et al. Ser. No. 830,145, "Improved Cellular Radiotelephone Land Station", by Atkinson et al.; and Ser. No. 830,390, "Interface Method and Apparatus for a Cellular System Site Controller", by Menich et al.) filed on the same date Feb. 18, 1986 as the present application and containing related subject matter.
The present invention generally relates to the fixed equipment of radiotelephone communication systems and more specifically relates to a method and apparatus for determining which antenna of a sectorized cellular radiotelephone system receives the strongest radio signal.
Mobile radiotelephone service has been in use for some time and traditionally has been characterized by a central site transmitting with high power to a limited number of mobile or portable units in a large geographic area. Mobile or portable transmissions, due to their lower transmission power, were generally received in previous systems by a network of receivers remotely located from the central site and the received transmission was subsequently returned to the central site for processing. In previous systems only a limited number of radio channels were available, thus limiting the number of radiotelephone conversations in an entire city to the limited number of channels available.
Modern cellular radiotelephone systems have a comparatively large number of radio channels available which, further, can be effectively multiplied by reuse of the channels in a metropolitan area by dividing the radio coverage area into smaller coverage areas (cells) using low power transmitters and coverage restricted receivers. Such cellular systems are further described in U.S. Pat. Nos. 3,906,166--Cooper et al.; 4,485,486--Webb et al.; and 4,549,311--McLaughlin, each assigned to the assignee of the present invention. The limited coverage area enables the channel frequencies used in one cell to be reused in another cell geographically separated according to a predetermined plan, such as a seven cell repeating omnidirectionally illuminated cell pattern shown in FIG. 1. In this pattern radio frequency energy is omnidirectionally transmitted from and received by a plurality of centrally located fixed stations and reuse of frequencies is accomplished in a pattern of cells such as that shown shaded in FIG. 1.
An alternative cellular pattern, FIG. 2, depicts a corner illuminated cell system in which 120.degree. antennas are employed to illuminate the interior of a cell from three of the vertices of a hexagonal cell. (Although cell systems are conventionally shown as regular hexagonal patterns, such regularity is rarely achieved in practice).
Another pattern, FIG. 3, depicts a center illuminated cell system in which the cells are further subdivided into sectors. The sectors are illuminated by 60.degree. antennas as illustrated in FIG. 3. A center illuminated sector cell system is further described in U.S. Pat. No. 4,128,740--Graziano and assigned to the assignee of the present invention. Thus, a large number of channels can be made available in a metropolitan area and the service provided thereby can appear to be identical to a standard wire line telephone.
A cell system typically utilizes one duplex frequency pair channel in each cell (a signalling channel) to receive requests for service from mobiles and portables, to call selected mobiles or portables and to instruct the mobiles or portables, to tune to another channel where a conversation may take place. This signalling channel is continuously assigned the task of receiving and transmitting data to control the actions of the mobile and portable radios. If the cell is sectorized as shown in FIG. 3, specialized receivers have been developed to enable the inputs from six 60.degree. antennas to be combined for instantaneous reception over the sectorized cell coverage area. One such specialized receiver is described in U.S. Pat. No. 4,369,520--Cerny, Jr., et al., assigned to the assignee of the present invention.
Since the cells may be of relatively small size, the likelihood of a mobile or portable travelling between sectors or out of one cell and into another cell is high. The process of switching the established call from one sector or from one cell to another is known as handoff. Handoff previously has required specialized receiving equipment such as a "scanning" receiver which can be instructed to tune to any of the channels in use in any of the sectors of the cell to measure the signal strength of each active mobile or portable. If the measured signal strength is below a predetermined level, cellular control equipment determines the availability of other channels in other sectors of the same cell or in neighboring cells and composes an instruction to the mobile or portable commanding it to tune to the new channel.
In order to determine which sector of a sectorized cell is receiving the best signal of the six sectors, traditional fixed site equipment included a plurality of specialized receivers which could be commanded to monitor one or more sectors at a particular frequency to determine whether a particular served remote unit signal was becoming too weak or whether a remote unit from another cell or another sector could be handed off to the particular sector being monitored. This traditional design required the specialized "scanning" receivers at additional cost and complexity.